1. Field of the Invention
The present invention relates to an improved machine for grinding the edges of sheets of plate glass, particularly for use in automobile windows.
It is well known that a sheet of glass which has been cut according to a desired profile, has to be ground at the edges, both to eliminate the cutting edge which otherwise could produce disadvantages during further working, and to remove possible peripheral microcracks which would cause breakage of the glass itself during the course of subsequent thermal treatment such as (tempering, bending, and the like.
The term grinding is used to define the operation which allows removal, by means of a diamond wheel, of a thickness of material located on the edge of the sheet for a depth which varies as a function of the use for which the sheet is intended.
2. Description of the Prior Art
Machines for grinding sheets of glass are already known from the prior art.
For example a type of apparatus used industrially is known in which the glass rotates around an axis of rotation around which also rotates a cam which is shaped like the glass. A follower wheel engages with the profile of the cam to regulate the position of the grind wheel with respect to the glass. This grinding system is commonly known as cam grinding. Examples of these machines are illustrated in U.S. Pat. Nos. 2,561,929 and 2,293,828, as well as published Italian application No. 19366A/79.
In all the machines illustrated in the above patents, it should be noted that the distance between the pivot point of the wheel arm and the axis of the shaft supporting the glass is fixed.
In French Pat. No. 2070521, which illustrates a manually controlled machine for processing profiles of sheets of glass at the artisan level, the distance between the axis of rotation of the wheel arm and the axis of the shaft supporting the glass is variable and, moreover the support of the wheel arm moves in a complex fashion, in any case not rectilinear with respect to the chassis.
U.S. Pat. No. 4,587,764 describes a grinding machine which does not use a copying cam, and uses instead an electronic control system which allows adjustment of the grinding parameters such as pressure and peripheral grinding speed, so as to obtain both the advantages of eliminating the copying cam and of obtaining a high working quality. This machine makes use of the combination of three mechanical motions: a rotational motion of the piece to be processed which defines the grinding speed, a rotational motion of the wheel arm about its pivot point which defines the grinding pressure, and a reciprocating linear motion of the pivot point itself in the horizontal direction, as a function of the shape of the piece being worked.
It has been noted in practical use that the above described system, while representing a considerable improvement with respect to the known systems, has some drawbacks in the case of grinding pieces having sharp corners of a very low radius of curvature. In fact in this case it is difficult to adjust the pressure and peripheral speed which are the characteristic parameters determining the grinding quality in the corner areas, resulting in an undesired deformation of the corner itself.